Nyob rau hauv cov ntaub ntawv sib txheeb, qhov kev vam khom tshwm sim thaum cov ntaub ntawv khaws cia hauv tib lub rooj database tshwj xeeb txiav txim siab lwm cov ntaub ntawv khaws cia hauv tib lub rooj. Qhov kev vam khom ntau yam tshwm sim thaum muaj ib lossis ntau kab hauv ib lub rooj qhia tias muaj ib lossis ntau lwm kab hauv tib lub rooj. Muab lwm txoj hauv kev, ob tus cwj pwm (lossis kab) hauv ib lub rooj yog ywj siab ntawm ib leeg, tab sis ob qho tib si nyob ntawm tus cwj pwm thib peb.
A multivalued dependency tiv thaiv tus qauv normalization thib plaub ib txwm daim ntawv. Relational databases ua raws tsib daim ntawv ib txwm uas sawv cev rau cov txheej txheem rau kev tsim cov ntaub ntawv. Lawv tiv thaiv kev hloov kho anomalies thiab inconsistencies nyob rau hauv cov ntaub ntawv. Daim ntawv plaub ib txwm cuam tshuam nrog ntau-rau-ib kev sib raug zoo hauv cov ntaub ntawv.
Functional Dependency vs. Multivalued Dependency
Yuav kom nkag siab txog qhov muaj txiaj ntsig ntau qhov kev vam khom, nws yog qhov pab tau kom rov mus saib seb qhov kev vam meej yog dab tsi.
Yog tias tus cwj pwm X tshwj xeeb txiav txim siab tus cwj pwm Y, ces Y yog qhov ua haujlwm nyob ntawm X. Qhov no tau sau ua X -> Y. Piv txwv li, hauv Cov Tub Kawm Ntawv hauv qab no, Student_Name txiav txim siab qhov loj:
| Student_Name | Major |
|---|---|
| Ravi | Art History |
| Beth | Chemistry |
Qhov kev cia siab ua haujlwm no tuaj yeem sau tau: Student_Name -> Major. Txhua Tus Tub Ntxhais Kawm_Name txiav txim siab ib qho loj thiab tsis muaj ntxiv.
Yog tias koj xav kom cov ntaub ntawv los kuj taug qab cov kis las uas cov tub ntxhais kawm coj mus, tej zaum koj yuav xav tias txoj hauv kev yooj yim tshaj plaws los ua qhov no tsuas yog ntxiv lwm kem hu ua Kev Ua Si:
| Student_Name | Major | Sport |
|---|---|---|
| Ravi | Art History | ncaws pob |
| Ravi | Art History | Volleyball |
| Ravi | Art History | Tennis |
| Beth | Chemistry | Tennis |
| Beth | Chemistry | ncaws pob |
Qhov teeb meem ntawm no yog Ravi thiab Beth ua si ntau yam kis las. Nws yog ib qho tsim nyog yuav tau ntxiv kab tshiab rau txhua qhov kev ua si ntxiv.
lub rooj no tau qhia txog kev muaj txiaj ntsig ntau vim tias qhov loj thiab kev ua kis las tsis muaj kev ywj pheej ntawm ib leeg tab sis ob qho tib si nyob ntawm tus menyuam kawm ntawv. Qhov no yog ib qho piv txwv yooj yim thiab txheeb xyuas tau yooj yim, tab sis ntau qhov kev cia siab tuaj yeem dhau los ua teeb meem hauv cov ntaub ntawv loj, nyuaj.
A ntau qhov kev vam meej yog sau X ->-> Y. Hauv qhov no:
Student_Name ->-> Major
Student_Name ->- > Sport
Qhov no yog nyeem raws li "Student_Name multidetermines Major" thiab "Student_Name multidetermines Sport."
Kev vam khom ntau yam ib txwm xav kom tsawg kawg peb tus cwj pwm vim tias nws muaj tsawg kawg ob tus cwj pwm uas nyob ntawm ib feem peb.
Multivalued Dependency thiab Normalization
Ib lub rooj nrog ntau qhov kev cia siab ua txhaum tus qauv normalization ntawm plaub daim ntawv ib txwm vim nws tsim cov khoom siv tsis tsim nyog thiab tuaj yeem ua rau cov ntaub ntawv tsis sib xws. Txhawm rau nqa qhov no mus txog 4NF, nws yog ib qho tsim nyog yuav tsum tau tawg cov ntaub ntawv no ua ob lub rooj.
Cov lus hauv qab no tam sim no muaj kev ua haujlwm ntawm Student_Name -> Loj, thiab tsis muaj kev cuam tshuam ntau yam:
| Student_Name | Major |
|---|---|
| Ravi | Art History |
| Ravi | Art History |
| Ravi | Art History |
| Beth | Chemistry |
| Beth | Chemistry |
Thaum lub rooj no tseem muaj ib qho kev ua haujlwm ntawm Student_Name -> Kev Ua Si:
| Student_Name | Sport |
|---|---|
| Ravi | ncaws pob |
| Ravi | Volleyball |
| Ravi | Tennis |
| Beth | Tennis |
| Beth | ncaws pob |
Normalization feem ntau ua tiav los ntawm kev ua kom yooj yim cov ntxhuav kom lawv muaj cov ntaub ntawv ntsig txog ib lub tswv yim lossis lub ntsiab es tsis yog sim ua ib lub rooj muaj cov ntaub ntawv sib txawv ntau dhau.
